1. Field of the Invention
The present invention relates to electronic equipment provided with a fuel cell, and a battery pack and a load apparatus used in this electronic equipment. Particularly, it relates to a suitable circuit configuration of a power-source section.
2. Description of the Background Art
Conventionally, electronic equipment such as a notebook computer and a cellular phone is provided with a plurality of voltage converters. It includes a power source which outputs several voltages, using a voltage converter which drops the voltage of a secondary battery or a voltage converter which boosts the voltage of the secondary battery. As the input power source of these voltage converters, the method of connecting them to the secondary battery has generally been adopted.
FIG. 10 is a block diagram, showing conventional electronic equipment such as a notebook personal computer on the market. A battery pack 400 is formed only by a secondary battery 102, not including a fuel cell. Both terminals 106, 108 of the secondary battery 102 are connected to both terminals 306, 308 of a load apparatus 300, respectively. In the load apparatus 300, the electric power inputted in four voltage converters 311 to 314 is all supplied from the secondary battery 102. The voltage converters 311 to 314 output voltages V1 to V4 after their conversion, respectively, to a function circuit 303.
In recent years, as the power source of electronic equipment such as a notebook computer and a mobile phone, a fuel cell has received attention which is capable of supplying electric power continuously for a long time. Such electronic equipment usually undergoes a sharp load fluctuation, while the electric power generated by a fuel cell cannot be rapidly changed. Hence, a secondary battery is charged with the power generated by the fuel cell, and simultaneously, the secondary battery supplies electric power to the electronic equipment. This is called a hybrid-type fuel-cell system, which has been offered in various forms. Among them, for example, Japanese Patent Laid-Open No. 2004-208344 specification gives the method of using a portable terminal which includes a fuel cell, a plurality of secondary batteries and a plurality of function circuits, so that energy utilization efficiency can be enhanced.
On the other hand, in order to charge a secondary battery with the electric power generated by a fuel cell, a voltage converter (i.e., a DC/DC converter) is required which converts the voltage of the fuel cell into the voltage of the secondary battery. In this case alike, as the input power source of a plurality of voltage converters, the method of connecting them to the secondary battery is in common use. Thus, such a method is proposed as controlling the fuel cell's output voltage so that it can be kept constant (e.g., refer to U.S. Pat. No. 6,590,370 specification).
FIG. 11 is a graphical representation, showing current-voltage characteristics according to the supply of fuel in a fuel cell formed by connecting six cells in series. In FIG. 11, the vertical axis indicates the output voltage (V) of a DMFC (or direct methanol fuel cell), and the horizontal axis represents the output current (A) of the DMFC. Reference characters and numerals C11, C12, C13 each denote a current-voltage characteristic curve if the total fuel supply is 0.6 cc/min, 1.2 cc/min, 1.8 cc/min, respectively.
As can be seen from FIG. 11, the larger the fuel supply becomes, the greater output current can be obtained. As shown by C11 to C13, the greater the output current becomes, the lower the output voltage will be.
In addition, if the fuel cell's output voltage is controlled so as to be fixed, as the supply of a fuel (i.e., methanol) is increased, the output current (A) rises. In the example shown in FIG. 1, if the fuel cell's output voltage is controlled so as to be kept at 2.4 V, in the case of the total fuel supplies like C11, C12, C13, each current (A) increases like I1, I2, I3, respectively. Therefore, the electric power generated by the fuel cell can be controlled by controlling the total fuel supply. In this way, in order to control the fuel cell's generation power using the fuel supply, it is desirable that the method be adopted of controlling the fuel cell's output voltage so that it is kept constant.
FIG. 12 is a block diagram, showing conventional electronic equipment in which a battery pack provided with a fuel cell is used. A load apparatus 300 shown in FIG. 12 is configured in the same way as FIG. 10. A battery pack 500 is configured by a fuel cell 101, a voltage converter 103 and a secondary battery 102.
Inside of the load apparatus 300, a voltage-converter group 301 is provided which is formed by four voltage converters 311 to 314 of 12 V, 10 V, 1.5 V, 1.25 V, respectively. The voltage converters 311, 312 of 12 V, 10 V are step-up circuits, and the voltage converters 313, 314 of 1.5 V, 1.25 V are step-down circuits. The electric power consumed by these four voltage converters 311 to 314 is all supplied from the secondary battery 102, as is the case with FIG. 10.
However, in a conventional hybrid-type fuel-cell system, it is difficult to supply electric power from a fuel cell to a function circuit whose load fluctuates sharply. This is because even if the flow rate of supplied fuel is changed, the fuel cell's output power does not vary rapidly. Judging from the fuel cell's output-current characteristic, the electric-current value of a load apparatus cannot suitably respond to a sharp change in its power consumption.
In addition, in the conventional electronic equipment shown in FIG. 12, for example, if the fuel cell 101's output voltage is 2.4 V and if the secondary battery 102's output voltage is 6 to 8.4 V, the electric power of the voltage converters 313, 314 of 1.5 V, 1.25 V is used after the following procedure. In a steady state, the voltage from the fuel cell 101 is boosted by the voltage converter 103, and then, the secondary battery 102 is charged. Thereafter, the voltage is dropped to 1.5 V, 1.25 V. In short, the fuel cell 101's output voltage is stepped up, and afterward, the voltage is stepped down. This causes a power loss, thus making such electronic equipment inefficient.
On the other hand, if the fuel cell 101's output voltage is 10 V and if the secondary battery 102's output voltage is 6 to 8.4 V, the electric power of the voltage converters 311, 312 of 12 V, 10 V is used after the following procedure. In a steady state, the voltage from the fuel cell 101 is dropped by the voltage converter 103, and then, the secondary battery 102 is charged. Thereafter, the voltage is boosted to 12 V, 10 V. In short, the fuel cell 101's output voltage is stepped down, and afterward, the voltage is stepped up. This causes a power loss, thereby making the electronic equipment inefficient.
In this way, in such a conventional hybrid-type fuel-cell system, from the secondary battery 102, electric power is supplied to the load apparatus 300. Hence, if the power-source voltage of the function circuit 303 is lower than the fuel cell 101's output voltage, for example, even if the function circuit 303 is a CPU circuit or the like, then the input of the voltage converter which supplies electric power to the function circuit 303 needs to be supplied from the secondary battery 102. As a result, after the fuel cell 101's output voltage is boosted to the secondary battery 102's output voltage, the secondary battery 102's output voltage is dropped to the function circuit 303's power-source voltage which is lower than the fuel cell 101's output voltage. Or, in contrast, after the fuel cell 101's output voltage is stepped down to the secondary battery 102's output voltage, the secondary battery 102's output voltage is stepped up to the function circuit 303's power-source voltage which is higher than the fuel cell 101's output voltage. Such an operation causes a power-conversion loss, thus raising an undesirable situation in respect of how to use energy efficiently.